1. Field of the Invention
The present invention relates to a method for manufacturing an R-T-B system rare earth permanent magnet containing, as main components, R (wherein R represents one or more rare earth elements, providing that the rare earth elements include Y), T (wherein T represents at least one transition metal element essentially containing Fe, or Fe and Co), and B (boron).
2. Description of the Related Art
Among rare earth permanent magnets, an R-T-B system rare earth permanent magnet has been increasingly demanded year by year for the reasons that its magnetic properties are excellent and that its main component Nd is abundant as a source and relatively inexpensive.
Research and development directed towards the improvement of the magnetic properties of the R-T-B system rare earth permanent magnet have intensively progressed. For example, Japanese Patent Laid-Open No. 1-219143 discloses that the addition of 0.02 to 0.5 at % of Cu improves magnetic properties of the R-T-B system rare earth permanent magnet as well as heat treatment conditions. However, the method described in Japanese Patent Laid-Open No. 1-219143 is insufficient to obtain high magnetic properties required of a high performance magnet, such as a high coercive force (HcJ) and a high residual magnetic flux density (Br).
The magnetic properties of an R-T-B system rare earth permanent magnet obtained by sintering depend on the sintering temperature. On the other hand, it is difficult to equalize the heating temperature throughout all parts of a sintering furnace in the scale of industrial manufacturing. Thus, the R-T-B system rare earth permanent magnet is required to obtain desired magnetic properties even when the sintering temperature is changed. A temperature range in which desired magnetic properties can be obtained is referred to as a suitable sintering temperature range herein.
In order to obtain a higher-performance R-T-B system rare earth permanent magnet, it is necessary to decrease the amount of oxygen contained in alloys. However, if the amount of oxygen contained in the alloys is decreased, abnormal grain growth is likely to occur in a sintering process, resulting in a decrease in a squareness. This is because oxides formed by oxygen contained in the alloys inhibit the grain growth.
Thus, a method of adding a new element to the R-T-B system rare earth permanent magnet containing Cu has been studied as means for improving the magnetic properties. Japanese Patent Laid-Open No. 2000-234151 discloses the addition of Zr and/or Cr to obtain a high coercive force and a high residual magnetic flux density.
Likewise, Japanese Patent Laid-Open No. 2002-75717 discloses a method of uniformly dispersing a fine ZrB compound, NbB compound or HfB compound (hereinafter referred to as an M-B compound) into an R-T-B system rare earth permanent magnet containing Zr, Nb or Hf as well as Co, Al and Cu, followed by precipitation, so as to inhibit the grain growth in a sintering process and to improve magnetic properties and the suitable sintering temperature range.
According to Japanese Patent Laid-Open No. 2002-75717, the suitable sintering temperature range is extended by the dispersion and precipitation of the M-B compound. However, in Example 3-1 described in the above publication, the suitable sintering temperature range is narrow, such as approximately 20° C. Accordingly, to obtain high magnetic properties using a mass-production furnace or the like, it is desired to further extend the suitable sintering temperature range. Moreover, in order to obtain a sufficiently wide suitable sintering temperature range, it is effective to increase the additive amount of Zr. However, as the additive amount of Zr increases, the residual magnetic flux density decreases, and thus, high magnetic properties of interest cannot be obtained.